Wednesday, 21 May 2014

‘UNSTOPPABLE COLLAPSE’ OF THE WEST ANTARCTIC ICE SHEET IS NOT HAPPENING

“Scientists Warn of Rising Oceans From Polar Melt” and goes on to say: “A large section of the
mighty West Antarctica ice sheet has begun falling apart and its continued
melting now appears to be unstoppable, two groups of scientists reported on
Monday. If the findings hold up, they suggest that the melting could
destabilize neighboring parts of the ice sheet and a rise in sea level of 10
feet or more may be unavoidable in coming centuries.”

The story, based on
studies of the Pine Island and Thwaites glaciers in West Antarctica, went
ballistic in the news media with dire predictions of the ‘unstoppable’
catastrophe about to unfold.

The authors of
the Pine Island paper, Justin Gillis and Kenneth Chang say in their paper: “…we find no major bed
obstacle that would prevent the glaciers from further retreat and draw down the
entire basin.” In a second paper, “Marine Ice Sheet Collapse Potentially Underway for the Thwaites
Glacier Basin, West Antarctica,” the authors (Joughin, Smith, and Medley) also infer
that the entire West Antarctic ice sheet will soon disappear, resulting in a
sea level rise of up to 10 feet.

Figure 1. Antarctica. The
East Antarctic ice sheet makes up more than 90% of Antarctic

ice and has been
growing. The West Antarctic ice sheet accounts for only 8½ % of

Antarctic ice
and the Pine Island glacier (red dot) makes up only about 10% of that.

(NASA
image)

WHAT’S
HAPPENING NOW

Since last week’s press
releases, thousands of Antarctic photos and text items on the internet have
been relabeled, and you can scarcely find any mention of Antarctica now without
reference to ‘the unstoppable, collapsing Antarctic ice sheet.’ From all of the
media hype, you would think that the West Antarctic ice sheet is presently in
the process of collapsing and drastic sea level rise is imminent. THE WEST ANTARCTIC ICE SHEET IS NOT
COLLAPSING! The retreat of the Pine Island and Thwaites glaciers is NOT caused
by global warming, and sea level is NOT going to rise 10 feet.

The two papers predict
that it could collapse in several hundred years, based on retreat of two outlet
glaciers that drain part of the ice sheet.The authors contend that recent retreat of the Pine Island and Thwaites
glaciers has occurred because warm ocean water has caused melting of ice on the
underside of the glaciers, causing them to thin and calve more rapidly. Because
the base of most of the West Antarctic ice sheet lies below sea level, the
authors contend that ocean water will melt its way up several small embayments
under the ice sheet, which is more than 1000 miles across, and cause it to
collapse abruptly.They refer to this as
“unstoppable” because the glacier base is below sea level and they claim that
there is nowhere that the glacier can ground so it will all collapse into the
sea.

Figure
2. Pine Island outlet glacier and the northwestern part of the West Antarctic
ice sheet.

(modified from Google Earth)

The prediction of
catastrophic collapse of the entire West Antarctic ice sheet is based on
several lines of evidence:

1.The Pine Island and Thwaites glaciers have shown recent increases
in calving and retreat of their termini.

2.The base of the West Antarctic ice sheet is mostly 1,000 m (3,300
ft) below sea level, so warm ocean water can cause melting of basal ice at the
terminus, resulting in calving and rapid retreat of the ice terminus. As the
terminus enters deeper water, it can retreat much faster.

3.As the ice retreats, there are no more grounding line positions,
so calving will accelerate and cause collapse of the ice sheet.

4.If a large sector of the West Antarctic ice sheet has gone into
irreversible retreat, the entire West Antarctic ice sheet will collapse,
raising sea level 10 feet.

These assertions are not new—36 years ago,
Mercer (1978) suggested that the West Antarctic ice sheet was potentially
unstable and others have commented on it before and since then. Here is what
some have said: Calving of
large icebergs is a natural process unrelated to warming–this ice shelf and
others spawn huge icebergs every 6-10 years. Releasing a huge iceberg, by
itself, is a normal process. Collapse of Pine Island glacier, if it did occur,
would take 1000-2000 years, but it is unlikely to contribute to more than 2.7
cm of sea level rise over the next 100 years. Every 10 years or so ice shelves
calve large icebergs, which are not worrisome. This ice stream is unlikely to
collapse in our lifetime.

THE BIG PICTURE--THE GEOLOGIC SETTING

To get a perspective of what
is happening now and what might or might not happen in the future requires a
look at the overall geologic setting and the scale of the size and thickness of
the West Antarctic ice sheet relative to the Pine Island and Thwaites glaciers.
The East Antarctic ice sheet makes up more than 90% of Antarctic ice. The West
Antarctic ice sheet (Figure 1) makes up only about 8½ % of Antarctic ice, and
the Pine Island glacier makes up about 10% of the West Antarctic ice sheet.
Most of the West Antarctic ice sheet lies SE of the Pine Island glacier and at
its SW margin is about 1000 miles from the Pine Island and Thwaites outlet
glaciers. Ice in the SE region flows into the Ross Sea, making the Ross Ice
Shelf, and has little if anything to do with the part of the ice sheet that
flows through the Pine Island and Thwaites outlet glaciers. The Pine Island and
Thwaites glaciers are not independent glaciers—they are ice streams from the NW
part of the West Antarctic ice sheet flowing through narrow embayments bounded
by mountains. Their termini calve into the Amundsen Sea, but the rest of the
ice sheet is grounded and all of the southwestern part discharges into the Ross
Sea (Figures 3,4). The entire western and southern margins of the West
Antarctic ice sheet are separated from the ocean by mountains so these are
virtually the only outlets for the ice. The total width of the Pine Island and
Thwaites outlet glaciers makes up only about 60 miles of the 2,500 miles of
coastline along the western and southern margins of the ice sheet. The major
ice discharge from the SW margin into the Ross ice shelf is not affected by what
goes on in the northern part of the ice sheet. Scale is important--only when
looking a map of the size of the West Antarctic ice sheet does it become
apparent just how tiny the Pine Island and Thwaites outlet glaciers are
relative to the size of the West Antarctic ice sheet (Figures 3,4).

Figure
3. West Antarctic ice sheet. (NASA image)

Figure 4. Location of the Pine
Island and Thwaites glaciers (red dots) and the West Antarctic ice sheet. The
ice sheet is bounded along its entire south coast by mountains so most of the
ice discharges into the Ross ice shelf and several narrow gaps in the mountains
where ice discharges into the Amundsen Sea. Note how much larger the ice sheet
margin is at the Ross Sea outlet than that of the Amundsen Sea outlet.
(Modified from Wikipedia, NASA).

The base of
most of the West Antarctic ice sheet lies below sea level (Figure 5) and it is
because of this that the West Antarctic ice sheet is predicted to collapse. The
deepest parts of the subglacial basin are mostly about 1000 m (3,300 ft) deep
and lie beneath the central portion of the ice sheet where the ice is the
thickest (Figure 6). More important than just depth below sea level is how
thick the ice is relative to the depth below sea level.

Figure 6 shows the
thickness of the West Antarctic ice sheet. The ice is more than 3,000 meters
thick in the darker red areas and most of the ice sheet is more than 2,000 meters
thick. The importance of ice thickness is that virtually all of the ice sheet
is considerably thicker than the depth below sea level to bedrock, so the ice
is grounded and will not float.

Figure
6. Thickness of the West Antarctic ice sheet. Contour lines are in meters, map
distance in kilometers.

(American Geographic Society)

Also important is the
source area of the outlet glaciers. Figure 7 shows ice divides and ice drainage
areas. The Pine Island outlet glacier drains only a relative small portion of
the West Antarctic ice sheet so it is difficult to see how events there could
result in collapse of the entire Antarctic ice sheet.

Figure 7. Ice
divides and ice drainages in the West Antarctic ice sheet. Light green is the
area of ice

draining into the Pine Island glacier; dark green is ice draining
into the Thwaites glacier; light and dark

ARGUMENTS AGAINST POTENTIAL COLLAPSE OF THE ENTIRE
WEST ANTARCTIC ICE SHEET

The authors assert that “…we find no major bed obstacle that would prevent the glaciers from
further retreat and draw down of the entire basin.” but that is contrary to what is
shown on Fig. 8.

Figure 8 is a profile of the West Antarctic ice sheet from the east coast
to the Transantarctic Mts., showing thickness of the ice sheet, sea level, and
the subglacial floor. At its deepest part, the subglacial floor is 2,000 m
(6,500 ft) below sea level, but almost all of the subglacial floor in this
profile is less than 1,000 m (3,300 ft) below sea level. The ice is mostly more
than 2,500 m (8,000 ft) thick, so basic physics tells us it will not float in
1,000 m (3,300 ft) of water nor will sea water melt its way under the ice.

1.At least half a dozen potential grounding lines may be seen in
Figure 8.

2.200 km (125 miles) up-ice from the terminus, the ice sheet is
about 1600 m (5,200 ft) thick and the subglacial floor is above sea level.

1.The Pine Island and Thwaites outlet glaciers
drain less than half of the West Antarctic ice sheet, so it is not apparent how
they could cause collapse of the entire ice sheet.

2.The Pine
Island and Thwaites outlet glaciers are only about 30 miles across so draining
2.2 million km3 of ice through their narrow channels or sending sea
water 1,000 miles under the ice sheet doesn’t seem plausible.

3.Most of the
ice sheet is much thicker (2,500 m (8,000 ft) than the depth of the subglacial
floor below sea level (1,000 m (3,300 ft) so the ice will not float and sea
water will not extend under the ice.

4.Even if the
ice sheet were to recede significantly, there are at least half a dozen
potential grounding lines at which the glacier would stabilize.

5.The West
Antarctic ice sheet is NOT collapsing, the retreat of these small glaciers is
NOT caused by global warming, and sea level is NOT going to rise 10 feet.